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如何解决ValueError: KivyMD应用需继承自kivymd.app.MDApp？

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DevOps 小满
2026-8-24
KivyMD热重载应用报错：ValueError: KivyMD: App object must be inherited from kivymd.app.MDApp

问题场景

使用Kaki/KivyMD开发热重载应用，运行live.py时触发上述报错，相关代码及报错回溯如下：

main.py代码 

from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.core.window import Window
from kivy.factory import Factory as F

Window.size = 360,640

class UI(F.ScreenManager):
    Builder.load_file("layout.kv")

class Main(MDApp):
     def build(self):
         return UI()

if __name__ == "__main__":
    Main().run()

live.py代码 

from kaki.app import App
from kivy.factory import Factory as F
import os

class Live(App):
    CLASSES = {
        "UI": "main"
    }

    KV_FILES = {
        os.path.join(os.getcwd(),"layout.kv")
    }

    AUTORELOADER_PATHS = [
        (".", {"recursive": True}),
    ]

    def build_app(self, first=False):
        return F.UI()

Live().run()

layout.kv代码 

<UI>
    Screen:
        MDLabel:
            text: "hello"

运行命令

set DEBUG=1 ; python live.py

完整报错回溯 

(venv) PS C:\Users\Cipher\PycharmProjects\AppliStage> set DEBUG=1 ; python live.py

[INFO   ] [Logger      ] Record log in 
C:\Users\Cipher\.kivy\logs\kivy_22-07-30_22.txt

[INFO   ] [deps        ] Successfully imported "kivy_deps.angle" 0.3.2
[INFO   ] [deps        ] Successfully imported "kivy_deps.glew" 0.3.1 
[INFO   ] [deps        ] Successfully imported "kivy_deps.sdl2" 0.4.5 
[INFO   ] [Kivy        ] v2.1.0
[INFO   ] [Kivy        ] Installed at "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\__init__.py"
[INFO   ] [Python      ] v3.9.10 (tags/v3.9.10:f2f3f53, Jan 17 2022, 15:14:21) [MSC v.1929 64 bit (AMD64)]
[INFO   ] [Python      ] Interpreter at "C:\Users\Cipher\PycharmProjects\AppliStage\venv\Scripts\python.exe"
[INFO   ] [Logger      ] Purge log fired. Processing...
[INFO   ] [Logger      ] Purge finished!
[INFO   ] [Factory     ] 189 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil (img_ffpyplayer ignored)
[INFO   ] [Window      ] Provider: sdl2
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] GLEW initialization succeeded
[INFO   ] [GL          ] Backend used <glew>
[INFO   ] [GL          ] OpenGL version <b'4.6.0 - Build 30.0.101.1340'>
[INFO   ] [GL          ] OpenGL vendor <b'Intel'>
[INFO   ] [GL          ] OpenGL renderer <b'Intel(R) HD Graphics 620'>
[INFO   ] [GL          ] OpenGL parsed version: 4, 6
[INFO   ] [GL          ] Shading version <b'4.60 - Build 30.0.101.1340'>
[INFO   ] [GL          ] Texture max size <16384>
[INFO   ] [GL          ] Texture max units <32>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
[INFO   ] [KivyMD      ] 0.104.2, git-bc7d1f5, 2021-06-06 (installed at "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivymd\__init__.py")
[INFO   ] [Text        ] Provider: sdl2
[WARNING] [Lang        ] The file C:\Users\Cipher\PycharmProjects\AppliStage\layout.kv is loaded multiples times, you might have unwanted behaviors.

 --- Logging error ---

 Traceback (most recent call last):
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kaki\app.py", line 163, in rebuild
     self.approot = self.build_app()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\live.py", line 19, in build_app
     return F.UI()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\screenmanager.py", line 973, in __init__
     super(ScreenManager, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\floatlayout.py", line 65, in __init__
     super(FloatLayout, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\layout.py", line 76, in __init__
     super(Layout, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\widget.py", line 366, in __init__
     self.apply_class_lang_rules(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\widget.py", line 470, in apply_class_lang_rules
     Builder.apply(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 540, in apply
     self._apply_rule(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 662, in _apply_rule
     self._apply_rule(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 658, in _apply_rule
     child = cls(__no_builder=True)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivymd\uix\label.py", line 324, in __init__
     super().__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivymd\theming.py", line 1129, in __init__
     raise ValueError(
 ValueError: KivyMD: App object must be inherited from `kivymd.app.MDApp`

 During handling of the above exception, another exception occurred:

 Traceback (most recent call last):
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\logging\__init__.py", line 1083, in emit
     msg = self.format(record)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\logging\__init__.py", line 927, in format
     return fmt.format(record)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\logger.py", line 291, in format
     record = copy.deepcopy(record)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 172, in deepcopy
     y = _reconstruct(x, memo, *rv)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 270, in _reconstruct
     state = deepcopy(state, memo)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 146, in deepcopy
     y = copier(x, memo)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 230, in _deepcopy_dict
     y[deepcopy(key, memo)] = deepcopy(value, memo)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 146, in deepcopy
     y = copier(x, memo)
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 210, in _deepcopy_tuple
     y = [deepcopy(a, memo) for a in x]
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 210, in <listcomp>
     y = [deepcopy(a, memo) for a in x]
   File "C:\Users\Cipher\AppData\Local\Programs\Python\Python39\lib\copy.py", line 161, in deepcopy
     rv = reductor(4)
 TypeError: cannot pickle 'traceback' object

 Call stack:
   File "C:\Users\Cipher\PycharmProjects\AppliStage\live.py", line 21, in <module>
     Live().run()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\app.py", line 954, in run
     self._run_prepare()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\app.py", line 924, in _run_prepare
     root = self.build()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kaki\app.py", line 101, in build
     self.rebuild(first=True)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kaki\app.py", line 168, in rebuild
     Logger.exception("{}: Error when building app".format(self.appname))
 Message: 'Live: Error when building app'
 Arguments: ()

 Traceback (most recent call last):
   File "C:\Users\Cipher\PycharmProjects\AppliStage\live.py", line 21, in <module>
     Live().run()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\app.py", line 954, in run
     self._run_prepare()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\app.py", line 924, in _run_prepare
     root = self.build()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kaki\app.py", line 101, in build
     self.rebuild(first=True)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kaki\app.py", line 163, in rebuild
     self.approot = self.build_app()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\live.py", line 19, in build_app
     return F.UI()
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\screenmanager.py", line 973, in __init__
     super(ScreenManager, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\floatlayout.py", line 65, in __init__
     super(FloatLayout, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\layout.py", line 76, in __init__
     super(Layout, self).__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\widget.py", line 366, in __init__
     self.apply_class_lang_rules(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\uix\widget.py", line 470, in apply_class_lang_rules
     Builder.apply(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 540, in apply
     self._apply_rule(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 662, in _apply_rule
     self._apply_rule(
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivy\lang\builder.py", line 658, in _apply_rule
     child = cls(__no_builder=True)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivymd\uix\label.py", line 324, in __init__
     super().__init__(**kwargs)
   File "C:\Users\Cipher\PycharmProjects\AppliStage\venv\lib\site-packages\kivymd\theming.py", line 1129, in __init__
     raise ValueError(
 ValueError: KivyMD: App object must be inherited from `kivymd.app.MDApp`

解决方法

核心原因

KivyMD的组件（如MDLabel）要求应用类必须继承自kivymd.app.MDApp，但当前live.py中的Live类继承的是普通的kaki.app.App（基于Kivy原生App），不满足KivyMD的依赖要求。

修改步骤

  1. 更新live.py代码
    将Live类改为多重继承MDApp和kaki的App，同时调整导入语句：
from kivymd.app import MDApp
from kaki.app import App as KakiApp
from kivy.factory import Factory as F
import os

class Live(MDApp, KakiApp):
    CLASSES = {
        "UI": "main"
    }

    KV_FILES = {
        os.path.join(os.getcwd(),"layout.kv")
    }

    AUTORELOADER_PATHS = [
        (".", {"recursive": True}),
    ]

    def build_app(self, first=False):
        return F.UI()

Live().run()
  1. 优化main.py的kv加载方式（可选）
    原代码中在UI类定义内加载kv文件会触发重复加载警告，建议将加载语句移到类外：
from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.core.window import Window
from kivy.factory import Factory as F

Window.size = 360,640

# 移到类外加载kv文件
Builder.load_file("layout.kv")

class UI(F.ScreenManager):
    pass

class Main(MDApp):
     def build(self):
         return UI()

if __name__ == "__main__":
    Main().run()

完成修改后，重新运行set DEBUG=1 ; python live.py即可正常启动热重载应用。

内容的提问来源于stack exchange，提问作者mathieu njembe

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最近更新时间：2026.08.24 16:24:21